This invention concerns a method and apparatus for feeding a liquid ferrous alloy to a mold made up of a gasifiable disposable pattern embedded in an unbound granular molding material, which prevents the appearance of the high-gloss carbon phenomenon.
In equipment of this kind, a gasifiable disposable pattern made of expanded polystyrene is used for each casting. Once produced, this pattern is covered with a heat-resistant coating and then placed in a mold where it is embedded in a granular material, generally sand, which is compressed by vibration. The mold is then fed with liquid metal, which vaporizes and displaces/replaces the gasifiable disposable pattern. This procedure is known by the general name of full mold casting.
When objects made of high fusion-point ferrous alloys such as cast iron or steel are manufactured, the material of the disposable pattern is not always completely vaporized. Furthermore, it frequently happens that the components of the disposable pattern, which have been vaporized in an initial stage, partially recondense later. In both cases, carbonaceous residues form at the surface of the molded part.
In the case of objects having a low carbon content, such as steels, the carbonaceous residues dissolve in the liquid metal. Even in this case, however, the consequences are not negligible because a carbonaceous gradient then appears in the object, whose surface is richer in carbon than its body.
Thus, objects made of a steel containing a low free carbon content could not, until now, be manufactured using the full mold casting process described above.
In the case of objects made of high-carbon steel, such as ductile cast iron, the consequences are even more pronounced. In fact, the phenomenon conventionally termed high-gloss carbon appears. It occurs randomly as discontinuities in the thickness and as irregularities in the surface of the molded object, and these discontinuities damage the object's solidity.
Thus, until now, there was no procedure or device making it possible to manufacture, simply and reproducibly, ductile cast iron objects possessing marked mechanical properties using the full mold casting process, which makes use of a gasifiable disposable pattern in expanded polystyrene or any other material which decomposes while producing carbon residues.
To compensate for the appearance of high-gloss carbon and for the diffusion of the carbon in the molded object, it has been suggested that the material of the disposable pattern be changed. For this reason, polymethyl methacrylate replaced polystyrene. When subjected to pyrolysis within the temperature range of the ferrous alloys, polymethyl methacrylate vaporizes completely but produces no carbon residues. However, while this product eliminates the emergence of high-gloss carbon, it poses other problems. Indeed, in addition to the problem of cost, polymethyl methacrylate appears to be less solid than polystyrene, thereby requiring increased precautions when handling the pattern, and furthermore, it generates a greater quantity of gas, leading potentially to a backflow of the liquid metal fed to the mold, thus doubling the mold filling time.